gst-env: spawn a shell to execute commands on windows

On windows, if you are not using built-in commands you need
to pass the full path of your executable into the subprocess.call/
Popen syscall. ex `c:/foo/bar/baz.exe`. This get's long and is
not ergonomic when you want to run trivial task like:

`gst-env.py ninja` or `gst-inspect0.0` or `gst-validate-launcher`

Instead, on windows, always launch a shell to be able to resolve
the executable from the PATH.

Plus another fix, although not 100% tested and sure its the correct behavior

gst-env: decode env var using utf-8
    
On windows python seems to be defaulting to cp1252, which becomes
problematic if the env vars contain non-ascii charaters.